Peritoneal exudate cells (PEC) from BALB/c mice harvested soon after the rejection of C57BL ascites leukosis EL4 are highly cytotoxic for EL4 cells in vitro. The cytotoxic reaction is specific, rapid, sensitive and highly reproducible. Heat stable serum components are essential for the reaction; albuminum, however, does not support cytotoxicity. The effector population is relatively homogeneous and is composed of small- to medium-sized pyroninophilic lymphoid cells which are non-adherent to nylon or plastic surface and appear to be non- dividing. Uridine is incorporated at the same rate as in control PEC from unsensitized mice, but the rate of incorporation of H3-thymidine is increased 5-fold over controls. It is proposed to attempt further purification of this uniquely effective killer cell population and to characterize as far as possible the actual killer cell, then to determine its fate, origin, and biochemical properties. These studies will form the basis for determining the mode of action of cell mediate cytotoxicity against ascites tumors.